The majority of existing head-up display systems are inserted in vehicles at manufacture, i.e. they are mounted and adjusted at the time of their construction. In this case, the vehicles are designed for this insertion to be performed under the best conditions in particular of positioning relative to the field of view of the driver. The adjustments, sometimes complicated, are performed at manufacture, on construction of the vehicle.
Configurations to be installed a posteriori as independent equipment insertable where necessary by the user also exist, but they are in the majority of cases very difficult to adjust, and/or not easily integratable in the vehicles in particular because the places in which they can be inserted are very limited.
One of the main reasons for these difficulties is associated with the fact that these systems include an optical component that is placed in the field of view of the user. The component provides optical functions including the correct positioning of the virtual image in this same field of view, functions of which the implementation with the technologies employed up until the present time causes the component to exhibit an insufficient degree of transparency with regard to the safety standards in force.
Such a system is intended to provide an additional functionality to improve the use of the vehicle, but must not at the same time remove a comfort element, detrimental moreover to driving safety.
In general, in the existing systems, the component placed in the field of view manages the projection depth, a sensitive parameter as it is linked to the integration of driving parameters of the vehicle in the environment that is the object of the visual attention of the driver. The depth is generally considered as having to be matched to the dimensions of the vehicle and the real scene (its value is generally greater than 1.80 m).
Similarly, the virtual image projected in the field of view of the driver should have a sufficient size and luminance, a compromise however having to be found to avoid disturbing the attention of the driver to what is happening around the vehicle while clearly displaying information useful to driving. Lastly, the positioning of the virtual image must be controllable in a vertical plane. These parameters together must make the reading of the virtual image comfortable both in relation to the real environment and in relation to the shape of the vehicle.
The head-up display systems currently available on the equipment market have a certain number of limitations in this respect mainly due to their technology. Thus, as they are based on holograms created for example from photosensitive materials or on semi-reflective minors, which is the case for the majority of currently available solutions, the levels of transparency obtained are low and in practice result in degrading of the field of view of the driver.
This level of transparency is in particular much lower than the minimum values required by the regulations associated with windscreens.
Secondarily, the level of the luminance that is obtained by means of these solutions is limited, also due to the insufficient transparency, which degrades the readability of the virtual image in practice.